The essentials of harmonic filtering techniques

High-pass filter at generator terminals

This example is aimed at illustrating the reduction of THDV using a high-pass filter in an installation. High-pass filter is used to control a resonant condition. The example describes another real-world application in which harmonic-related problems arose and even damaged a generator unit. This case presented an excellent opportunity for conducting investigations on a complicated phenomenon.

The AC source, a 3-MVA synchronous turbo generator, was the power supply for two 1.5 MVA step-down transformers, which in turn fed a number of VFDs powering downhole electrosubmersible pumps. Apart from a small service transformer that sourced the platform services, the VFDs were the only loads as illustrated in Figure above.

A number of harmonic measurements indicated that voltage and current harmonic levels were excessively high. The suspecting element was a 0.27-µF surge protection capacitor bank, which apparently combined with connecting cables between generator and transformers to excite a resonant condition at the generator bus.

Through additional measurements and simulation work, this was further confirmed to be the case.

Here, we will show the resonant condition and how it was controlled using a high-pass filter to reduce harmonic distortion levels at the generator terminals within IEEE-519 limits.

Figure 2 shows the abrupt increase in impedance (dotted line) revealing a resonant condition around harmonics 39 through 43.

Such a situation imposed severe stresses on the generator that comprised intense shaft vibration and increased operation temperature. Also shown is the Z–f characteristic of the band-pass filter that was applied at the 600-V generator bus. Filter elements were selected to obtain a corner frequency centered at around the 11th harmonic.

Notice the system response with the high-pass filter showing a substantial reduction of the impedance at the resonant point.

Figure 3 shows how the impedance frequency characteristic looks at the primary of one of the downstream transformers feeding a VFD at one of the oil wells.

Interestingly, the resonant phenomenon, as observed in Figure 6.14, involved a frequency range in which characteristic harmonics show very small (but apparently strong enough) values that, after undergoing amplification, made THD levels soar.

It is important to mention that applications of high-pass filters at the generator bus are usually combined with the application of single-tuned filters at the VFD locations to get THD levels within recommended limits at the generator terminals as well as at the individual VFD sites.

Leaving the high-pass filter at the generator bus as the only harmonic mitigating method may cause excessive heating on the high-pass filter elements.

In this example, the real intention of the high-pass filter was to eliminate the large impedance resulting from the resonant condition more than to provide a low-impedance path to all harmonic currents generated at the various VFD sites.